1. Field of the Invention
The present invention relates to a riser system which is used in subsea well completion and intervention operations to, among other things, provide a tubular conduit between the offshore drilling/intervention vessel and the subsea well. More particularly, the invention relates to a monobore riser system which is capable of providing selective communication between the surface vessel and the production and annulus bores within the wellhead.
2. Description of the Related Art
Riser systems are used in subsea well completion and intervention operations for installing, retrieving and intervening tubing hanger/completion strings and subsea xmas trees. Riser systems for conventional subsea completions comprise two tubular, typically steel conduits extending between the offshore drilling or intervention vessel and the subsea equipment. Simpler monobore casing risers, which are different from the subject invention, are typically used for horizontal xmas tree applications. These conduits represent the structural portion of the riser system and also allow for the transfer of fluids and wireline tools between the vessel and the production and annulus bores in the wellhead. Riser systems also include one or more controls umbilicals, which are typically a bundle of hydraulic hoses and electrical cables which transfer hydraulic and electrical power and control signals between the vessel and the subsea equipment in order to facilitate control of the subsea equipment from the surface. The combination of the structural riser conduits and umbilicals, and the specialty equipment related to each, is typically referred to as a completion/intervention (C/IR) riser system.
The prior art dual-bore riser systems employ two tubular conduits in one configuration or another, with each conduit providing direct communication between the surface vessel and either the production bore or the annulus bore within the wellhead. One riser configuration comprises pre-unitized joints of side-by-side production bore tubing and annulus bore tubing locked together by clamping elements. The bulk of the riser string is made up of typically 45 to 50 foot (and sometimes longer) lengths of these joints, although additional pup joints of varying shorter lengths are usually needed to adjust the final space-out between the surface vessel and the subsea equipment. The clamping elements provide the additional capability of securing the umbilicals to the conduits. Other riser configurations include individual strings of production bore and annulus bore tubing and various cased multibore and concentric bore designs.
The C/IR system may be used inside a conventional marine drilling riser or in an open sea environment. In the latter case, the riser may be deployed from an anchored or dynamically-positioned drilling rig or, alternatively, from a lighter weight, typically dynamically-positioned, service vessel. When used in the open sea environment, substantial loads are imposed on the riser and its deploying vessel. Consequently, the riser system should include a riser safety package (RSP) and an emergency disconnect package (EDP) to terminate the lower end of the riser and provide the necessary well control and safety features. Horizontal xmas trees do not normally require an "open sea" riser application except for the "light weight intervention" scenario. The subject invention provides the same benefits for horizontal xmas trees and conventional xmas trees under these circumstances (most notably in the area of annulus conduits).
Because the lighter-weight service vessels do not usually have the same storage and load-carrying capacity as drilling rigs, current C/IR systems cannot readily or practically be deployed from these vessels. Furthermore, even conventional drilling rigs are limited in their ability to deploy some riser systems effectively in very deep water applications because string weight can be a problem for tubing hanger landing and orientation operations. Also, as the water depth in which subsea wells are completed increases, both the capital and operating expenses associated with the riser system are likely to increase because more riser will be required and that riser will be exposed to greater forces, factors which will likely drive up the size and cost of the structural conduits, umbilicals and other components of the riser system.